I have air vents and an I-beam right when you walk into my basement. My family is tall so I am not looking to box it like I normally would. I am going to hit everything with white paint. Problem is on the outer walls how do you "connect" the drywall to the I-beam and vent that run across?
I'm not sure if I'm following all the details, but it sounds like one question is how do you finish the drywall around the I-beam where it will enter the sheetrock.
I did that in my basement, and a bit of caulk was all it needed between the sheetrock and metal. FWIW, consider painting the I-beam a bold color. Make it part of the "look".
As for the air vents, I'm not sure quite how that fits into the big picture. Some drawings/photos may help here.
Interesting problem. If you've got metal vent, and a steel I-Beam, those will definitely move at different rates than drywall on wood framing. Sounds like you need something flexible for the transition. What about using self-adhering flexible membrane flashing material as (at least) part of the solution?
I know you can get flex membrane flashing with a white surface, but I don't know if any manufacturers offer a paintable product.
Also, I'm not sure how to make a smooth transition from the membrane to drywall, but it might be possible with some careful mudding. I haven't actually used membrane for this solution, but I am a big fan of the product for other purposes. Here's a discussion article that mentions many types.
Let us know what you end up doing?
Why can't you box it in and use 1/4" plywood for the bottom of the box? That would only lower the ceiling height 1/4", which is not that much. (Plus if you're going to bump your head, better to be on some plywood and drywall than a steel beam.)
Normally ducting and I-beam are not more than two feet wide combined. If the width is 24 inches or less then frame vertically from ceiling where the bottom of frame-wall is flush with the bottom of I-beam and ducting. Fasten sheet rock to the bottom of the drop walls on each side of the ducting. This way the thickness of the drywall will be the only additional drop.
Extra deep U-channel affixed more or less rigidly to the vent and beam, with the dry wall free floating in the channel. Don't create a paint-seal between the drywall and the U-channel or it'll look uglier than whatever when it shifts over time, so cut in with at least a coat of primer on the dry wall before assembly.
You will likely need a forklift for this project, likely a big fork lift.